The Geekworks:10GHz White Box Conversion Notes

These are some notes on my project (together with other members of the
Midwest VHF/UHF Society) to convert the
MaCom 10GHz "White Box" (or "Whitebox" for the search engines) unit for ham
radio use. Since we're following in the path of others who've already
modified these units, there isn't much new here in the way of modification
instructions, but I've done a lot of evaluation of the unit and hopefully
the performance data presented here will be useful.

There are two source documents that virtually everyone converting the White
Box uses. The first is a paper by WA6CGR that was published in the 1991
Microwave Update. It's available several places on the web, but I have
put a copy of
Dave's paper (in PDF format) here.

These documents are interesting to compare because each focuses on doing
fairly extensive mods to different parts of the White Box unit. But neither
one does much in the areas that the other hacks on.

You can apply mixer theory to these papers and do a conversion that's the
sum of the changes both Peter and Dave made (i.e., lots of work), or the
difference (i.e., not much work at all).

Our group took the simple approach and didn't do any major modifications.
We just retuned the existing circuits, and used "high side" LO injection
which avoids a major retuning of the local oscillator module at the cost of
living with a reversed sideband and tuning direction compared to those who
use the more complex LO modification.

Frequency Stability and LO Issues

I've spent most of my time on this project characterizing and optimizing the
local oscillator's frequency stability. See
http://www.febo.com/time-freq/whitebox for details of my experiments,
and plots of the LO frequency stability.

LO Power Supply

The LO requires 18-20 volts for operation. Since I want to take my 10GHz
rig out on the road, I'd like to be able to run the whole system from a 12
volt power supply.

Again, both the WA6CGR and G3PHO papers have different approaches to doing
this. 'CGR suggests a circuit using an LM317T audio amplifier module as
an oscillator running at 20kHz. Dave then rectifies the output to provide
about +24 volts, which is regulated down to 18V. A separate rectifier driven
from the same signal provides -5 volts for the transmit amplifier bias.

G3PHO suggests a circuit using an LM2577-ADJ step-up regulator to generate
18 volts, and a separate 5 volt regulator driving a 7660 voltage inverter
to generate the bias requirement. (I've attached a
circuit diagram of his design here.)

I first built the LM317T circuit, but found that it was very inefficient
(well under 50%) and also had a lot of high frequency noise on the output.
I figured that in the end I'd figure out how to filter the noise, but the
temperature of the LM317T heatsink convinced me this wasn't the way to go.

Then I built the G3PHO design, and am very happy with it. I'm measuring --
hard as it is to believe -- greater than 90% efficiency and the LM2577
heatsink remains stone cold after hours of operation. The current drain
from a 13.8V supply is about 375ma (when the crystal heater is at operating
temperature -- the starting current is about 700ma but drops pretty rapidly).
By contrast, my LM-317T drew over 800ma and the idle current is quite high.

The LM2577 circuit uses one somewhat expensive component, a high-quality
ferrite coil that provides energy storage. Peter's design used a 100uH
coil, but my back-of-the-envelope calculation from the data sheet showed
that a 300uH coil was more in line. I used a Schott 67127080 (available
from Digi-Key) which cost about $13.00.

By the way, here's a construction hint: The Schott coil has two windings,
but this circuit uses only one of them. I initially paralleled the windings
in the hope of reducing series resistance. The power supply didn't work.
When I separated the windings, it took right off. My best guess is that the
windings have opposite phase. So, if you build this circuit with this
component, just use one of the windings and let the other one float.
(I had the clever thought that I could use the second winding as a transformer
secondary and pull voltage from it to provide -5 volt bias. Although I could
get plenty of voltage at no load, the winding can't product any significant
current without the voltage sagging to almost nothing, so I gave up on the
idea.)

Even though I added a few extra bypass capacitors on the output of the
supply, the 52kHz switching frequency still showed up as transients on the
output. These were strong enough to create sidebands on the LO output at
+/-52kHz. While the ones shown in the picture below are about 50dB down,
that's because I could only partially back out the fix for this picture;
originally, the sidebands were more like -38dB:

I added a 22uH power inductor in series with the output, and that knocked the
sidebands down to almost nothing:

LO Automatic Frequency Control Input

The local oscillator has an AFC input which (a) creates noise if it's left
floating, and (b) may need to be biased to allow the crystal to tune to the
correct frequency. I built an AFC bias circuit on the power supply board.
It's simply a 12 volt zener diode driven from the regulated +20 volt output,
and fed into a 10k cermet pot which allows fine frequency adjustment. To
minimize sidebands, I also put a 22uH inductor in series with the AFC output,
but it made no noticeable difference and probably isn't necessary if you
adequately filter the main supply.

Figuring out the right voltage to put on the AFC pin has been a bit of a
challenge. G3PHO uses a 3.3 volt zener, but we found that our units
ether wouldn't net to frequency, or start oscillating reliably (or both),
with that voltage. I suspect that different crystals may require different
AFC voltages to land in the right spot. I'm running about 9 volts and that
seems to put the trimmer cap near the middle of its range to yield the
desired frequency.

Further LO Improvements

We noted that the LO is very sensitive to supply voltage -- a 1 volt
supply change results in a 40kHz change of output frequency. G3PHO suggested
that separately regulating the oscillator transistor inside the LO could
reduce a slight frequency shift he noticed switching between RX and TX,
and I decided this might help the overall stability of the unit. Rather
than using a 3 terminal regulator as G3PHO did, I used a 15 volt zener diode.
The installation is very simple.

If you open the top of the LO and look in the corner near the frequency
adjust capacitor, you'll see two resistors mounted near the bottom edge of
the board (with the box aligned so the crystal is in the upper left quadrant,
they're mounted running north-south with the trimmer cap above and to their
left). One of them is 100 ohms. Remove that resistor and install one end
of a 330 ohm resistor in the now-empty hole near the bottom of the board,
and a short piece of insulated wire in the other hole.

Mount a solder lug on the mounting screw at the lower left corner. Get a 15
volt, 1 watt zener diode and mount a bypass cap of your choice (I used a 1.5
uF tantalum, which on reflection may not be ideal) across the diode. Solder
the anode end of the diode to the ground lug, and tie the cathode, the free
end of the 330 ohm resistor, and the free end of the wire together. Close up
the box, because you're finished.

With this mod, the LO wiggles only slightly when the supply voltage changes
from 18 to 20 volts. Here's a graph showing the frequency stability with
voltage, both when the oscillator (Q1) is regulated with a 15 volt zener and
when it isn't; it's easy to see the point at which the zener network stops
regulating (when I did the original unregulated test I didn't go below 17
volts; based on the regulated behaviour, I expect the curve would remain
similar if you extended the voltage downward):

Signal Purity

In general, the Whitebox LO does a good job; discrete spurious signals are far
enough down that they shouldn't cause any problems. There is one anomaly,
though.

When operating the LO on 18 to 20 volts, there is a very noticeable wideband
"hump" in the frequency output at about 400kHz above and below the desired
signal (there is an external attenuator in my test setup, so add 10dB to the
reference to get the actual level):

Lowering the the supply to 16.55v drops the level of the primary signal
by about 1dB, increases the noise level by nearly 10dB, and broadens it
as well (there's no longer a quiet area close to the carrier frequency).
There's clearly no reason to run at this voltage level:

But dropping the supply just a tiny bit more, to 16.34 volts, makes a
dramatic difference, at the cost of another .7dB or so in output level:

Until we do some on-air testing, we won't know whether there's any benefit to
running at this lower voltage. The question is whether the lower output
will reduce the power/sensitivity of the transverter, and what impact the
lower voltage has on frequency stability. Since the oscillator is regulated
at 15 volts, the regulation may still hold with a 16.3 volt supply if the
value of the series dropping resistor is reduced, but I haven't tested this.
(Note, by the way, that the oscillator may not start reliably with a supply
below about 17 volts -- once running, the PLL hangs in there down to 14 volts,
but it won't start up at that voltage.)

Other Conversion Tidbits

The Bias Supply

The -5 volt bias supply for the transmit converter has proven to be another
problem. As mentioned above, Peter's circuit uses the ICL7660 voltage
inverter chip to turn +5 volts into -5. The 7660 is rated for 20ma output,
but will have about 0.5 volt drop when the current is 10ma. You can parallel
multiple units to increase the output capacity. I built my circuit with two
7660s in parallel, figuring that would provide more than enough current.

It didn't. The bias current in standby mode is about 27ma (which caused the
output from my supply to sag to about 4.3 volts). On key down, the current
is more than 90ma and the voltage of the parallel 7660s sagged to only a bit
more than 2 volts. I'm not sure why the bias current is so high, but this was
verified on two different White Box converters.

After browsing some semiconductor manufacturer's sites on the web, I decided
to use a Maxim MAX764 DC-DC inverter for the -5 volt supply. This
chip can provide up to 300ma at -5V from a +12 volt input. It's claimed to
be 80% or more efficient. The circuit seems to work well, and holds at
-5 volts plus or minus a few millivolts through the normal range of loads.
The output seems to be very clean, except for a very low frequency (10Hz or so)
sawtooth with an amplitude of 30mv or so -- that's small enough that I didn't
lose any sleep over it.

Interfacing

I built a really simple and very clever T/R sequencing circuit that I found
in the
VHF/UHF DX Book by G3SEK. There's a slightly earlier version
of the schematic on-line at
http://www.ifwtech.co.uk/g3sek/dx-book/sequencer/. It may seem old
fashioned to use relays these days, but the circuit is simple and does
the job. With the relays I used a 470uF cap provides about a 40 millisecond
delay between the "fast" and "slow" contacts.

Here's a timing chart showing the delay on key and unkey for the +12RX,
+12TX, and external coax relays:

This circuit has more delay than is needed for the coax relays I am using
(which switch in about 6ms), and in particular the design provides unneeded
delay on unkey. However, with only about 35ms of delay on keying and 10ms
more than that on unkey, the delay isn't really an issue. Removing the unkey
delay would require an extra set of relay contacts that aren't there. If I
ever take the box apart I may reduce the timing cap to 270 or 330uF, but I
think the current circuit is "close enough for government work."

I added one extra safety feature to my T/R switching -- I put a SPST,
normally open, 5 volt reed relay in series with the keying line. The relay
is driven from the bias supply. If there's no bias, the keying line is
disconnected and there's no way to apply +12V to the transmit side of the
White Box converter. Better safe than sorry, I say!

IF Drive and Power Output

The White Box has a sensitive input circuit. -20dBm at 146MHz
seems to be about the right level to drive the exciter to maximum linear
output. Above that level, the gain starts to compress and the output
is fully saturated at -15dBm drive. I'm getting about +21 dBm (a bit more
than 100mw) saturated output.

I was lucky enough to acquire a MA/Com power amplifier that is rated for 1
watt (+30dBm) output. Mine will do about +31dBm of saturated output. The
nominal drive level is +20dBm, but it looks like mine may have more gain than
specified as it hits rated output with about +17.5dBm drive. Looking at the
system as a whole, at the IF input about -27dBm is the maximum drive level
for a clean signal, as the output starts to compress very rapidly above that
point. (The good news from this data is that I can afford 3dB of feedline
loss between the exciter and the PA and still get 1 watt output by increasing
the IF drive level to compensate.)

Here's a graph showing power output of both the exciter and power amplifier
for different drive levels:

I'm using a Yaesu FT-817 as the IF rig. The '817 can operate either on 13.8
volts or on an internal battery pack. It's rated for 5 watts output on high
power, but defaults to a medium power setting that (on my unit at least) puts
out almost exactly 2.0 watts at 146MHz. One very nice feature is that this
power output remains very stable over a wide range of supply voltages --
switching between a somewhat-discharged internal battery at 9.5 volts and
external power at 13.8 volts changes the output only a tiny bit.

The FT-817 hooks to a
Down East Microwave AOS-144
switching unit which combines a high-power attenuator and an RF-sensed
switching circuit to split a single RF input to separate RX and TX outputs
suitable for connection to a transverter. Two watts from the FT-817 yields
about +7.9dBm at the AOS-144 TX IF output. A home-brew attenuator in the TX
IF line provides about 29dB attenuation, so I have about -21dBm going into
the converter.

Summary

(24 June 2002) John, N8VZW, and I were both at the
Bellbrook Amateur Radio Club "Field Day"
station this weekend and we made a contact on 10GHz -- my first -- across
a couple of hundred yards of parking lot. It was good to confirm that the
rig really works!

I don't expect much from the receiver at this point, since there's no preamp
in front of the input filter and mixer. We have some WB5LUA preamps on
order from Down East Microwave
and the system won't be complete until one of those is in the front end.
A minor problem with any receiver testing is that I don't have a 10GHz
signal generator, so I'm going to have to go by on-air experience to see
how the RX is working.